Fix identity for aws_secrets_manager_resolver

Related: gitlab#557976 (closed)

Documentation update: gitlab#542366 (closed)

What does this MR do?

When aws_role_arn isn’t set, it should fall back to the default AWS credentials, see aws_secrets_management documentation

Why was this MR needed?

To fix the runner aws_secrets_manager_resolver when aws_role_arn is not specified.

What's the best way to test this MR?

1. Install the GitLab Runner with the applied fix.
2. Ensure your self-hosted GitLab instance is version 18.2 or higher.
3. Create the app-secrets/database secret in AWS Secrets Manager and a field password.
4. Provision an IAM role and assign it to your GitLab Runner instance.
5. Set up a test project using the .gitlab-ci.yml file provided below:

variables:
  AWS_REGION: us-east-1
fetch-password-via-aws-cli:
  stage: test
  script:
    - echo "Fetching DB password using aws cli from Secrets Manager..."
    - 'export SECRET_JSON=$(aws secretsmanager get-secret-value --secret-id app-secrets/database --region "$AWS_REGION" --query SecretString --output text)'
    - 'export DATABASE_PASSWORD=$(echo "$SECRET_JSON" | jq -r ".password")'
    - 'echo "Password: $DATABASE_PASSWORD"'
database-migration:
  secrets:
    DATABASE_PASSWORD:
      aws_secrets_manager:
        secret_id: app-secrets/database
        field: 'password'
      file: false
  stage: deploy
  script:
    - 'echo "DB PASSWORD"'
    - 'echo "$DATABASE_PASSWORD"'

The first scenario, fetch-password-via-aws-cli, should succeed in all cases — with or without the fix.

The second scenario, database-migration, should succeed only after updating the runner with the fix.

What are the relevant issue numbers?